Ever have that issue where you are trying to connect to a serial device with a USB serial to console cable but the ends don't connect - if you're smart you might have adapters in your serial kit to get around this issue. But if you're like me and a little less than prepared, you might be out of luck ... that is unless you hoard cables and happen to have a couple old Cisco terminal cables and some ethernet jacks laying around
Introducing the over engineered Null Modem Cisco cable adapter!
The way it works is pretty simple math:
A standard Serial DB9 to RJ45 Null modem connection requires the following pin mapping.
Each Cisco cable is wired this way but when you connect two of them together the pinouts cancel each other out which technically makes it a standard straight through serial connection, so in order for this hack to work the RJ45 ends needs to re-enable this pinout.
So I went to work and carefully mapped the RJ45 ethernet jacks as shown, then connected each end of the Cisco cable to the USB to serial adapter connected to my computer and serial port on the target device. I was amazed to discover it worked, I now had local console access and could interact with the device.
In the meantime I was looking on line and there are several inexpensive serial adapters available for order from the local electronics store that are intended to avoid all this hassle, but it wasn't really an option as I had limited time to complete the necessary work and couldn't leave my desk to go get them.
This was a super fun project that helped me solve a work problem and let me think outside the box a little. I would recommend trying this hack out only if you're in a similar pinch and happen to have old cisco cables and ethernet jacks kicking around. I ended up purchasing the correct adapters after the fact, and who knows when I'll need them again, but chances are I'll never forget to keep them handy in case I do.
